This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Inhibitors of apoptosis (IAPs) are anti-apoptotic factors that block cell death in response to a variety of stimuli. IAP family members contain a variable number of protein-protein interaction domains in addition to a C-terminal RING domain that confers E3 ubiquitin ligase activity to the IAP proteins. Binding of small molecule antagonists to select baculovirus IAP repeats (BIR) within cellular IAPs (cIAPs) promotes cIAP auto-ubiquitination and subsequent proteasomal degradation, thereby releasing cIAP inhibition of apoptosis. The means by which this binding event in turn influences E3 ligase activity within the distant RING domain remains unclear. One simple hypothesis is that antagonist binding causes the cIAP protein to undergo a structural rearrangement from a "closed" to an "open" conformation, which is then competent to form active RING-RING dimers. In the "closed" state this dimer interface is inaccessible and active dimers cannot form. In support of this hypothesis, we find that addition of antagonist to purified, cIAP1 monomers causes dimer formation, as monitored by native gel electrophoresis, size exclusion chromatography and dynamic light scattering measurements.